The Duke of Sussex stepped back from his senior royal role last year alongside his wife Meghan Markle.
However, royal biographer Angela Levin said Diana would have told her youngest son “to be there for William as he gears up to be King”.
What’s the latest news on Prince Harry?
Ms. Levin told OK! Magazine: “Diana would be totally heartbroken. It was so important to her that her boys had a good relationship.
“They had such a unique life and so she wanted to ensure they’d at least have each other.”
She added: “If Diana were here now, she’d give Harry a telling off for breaking away, and she’d tell him to be there for William as he gears up to be King.”
What’s happened between Harry and William?
Rumors of a rift between Harry and Prince William have existed for years.
Tensions apparently began when the Duke of Cambridge reportedly told his brother to take things slow with Meghan.
Things have now become worse since Harry and Meghan stepped back from royal life, and made explosive claims about the royal family during their Oprah interview.
Earlier this year, Harry spoke about his relationship with William during his interview with Oprah Winfrey.
He said: “As I’ve said before, I love William to bits. He’s my brother.
“We’ve been through hell together. We have a shared experience, but we were on different paths.”
Meanwhile, in April, the brothers reunited at their grandfather Prince Philip‘s funeral.
Royal fans were delighted when William and Harry were spotted walking and talking together following the service.
The brothers are expected to reunite next month for the unveiling of a Princess Diana statue.
The statue is being put up at Kensington Palace to honor what would have been Diana’s 60th birthday.
Royal commentator Richard Fitzwilliams said it will be a “deeply personal moment” for Harry and William.
Mr. Fitzwilliams added to ED!: “Both cherish memories of their mother, who so tragically died in that fateful car crash and both, especially Harry, have been deeply affected by her death.”